Selenium is a possible cancer prevention agent, and is being considered for use in intervention trials. Two studies in collaboration with the Cancer Prevention Studies Branch, DCPC Prevention Program, are planned to provide information on the bioavailability and pharmacokinetics of selenium in its prototype forms-sodium selenite (inorganic form) and selenomethionine (organic form). This information is unavailable for these agents in the dose currently considered optimal, and is necessary to the determination of time and manner of administration. The initial study will be the Selenium Pharmacokinetic Study. Parameters such as percent absorption, maximum concentration, time to maximum concentration, and half-life will be estimated for a single dose and compared in both fasting and nonfasting subjects. A second study, the Selenium Chronic Supplementation Study, will examine and compare the effects of multiple dosing on the absorption, distribution, and excretion of these two forms of selenium and a placebo. The Operations Research Branch, in cooperation with the Cancer Prevention Studies Branch, will function as a data collection center and has primary responsibility for the study design and analysis.